Physical Sciences Research Highlights

May 2016

It Takes Two to Make an Electrode Go Right

Antimony fails to work inside a magnesium battery, but it's just what tin needs to store energy

In a tin and antimony alloy, a potential electrode for magnesium batteries, the metals separate when the battery is first charged. Both metals bind with magnesium ions, but only the tin regions release them. However, the antimony serves a vital role in stabilizing the overall structure. Image: Cortland Johnson, PNNL

With more capacity and fewer safety
issues than their lithium counterparts, magnesium batteries are potentially a promising
energy storage option, but the electrodes are difficult to produce and quickly
fail. Scientists want something better. Inspired by a two-metal electrode, made
of tin and antimony, a team at DOE's Pacific Northwest National Laboratory (PNNL)
delved into the atomic workings of this alloy. They saw the metals separate
into antimony- and tin-rich regions. The tin regions worked well; the
antimony-rich areas did not. However, the antimony regions were crucial: at the
interface, or border, between the two regions, the antimony kept the tin
structure from collapsing.

This work has applications beyond
batteries. "More generally, this research is developing new knowledge on how to
synthesize materials with certain properties and certain structures," said Dr. Maria
Sushko, computational physicist at PNNL. "The Office of Science at DOE provided
the environment to do research in such broad areas."

Why It
Matters: One of the
concerns with electric cars is long, lonely stretches of highway. Drivers don't
want to be stranded between charging stations. This concern can factor into
their decision to buy lower emission vehicles. The results of this study add to
the scientific understanding of how to create materials for better magnesium batteries
and other energy storage devices.

"This research offers important information on how to start with an
alloy and create a phase-separated material with a completely different
structure that is useful for energy technologies," said Dr. Yuyan Shao, a chemist
on the team.

Methods: In
batteries, certain ions must move in and out of the electrode with each use. If
an electrode fails to welcome or release the ions, the battery fades and fails.
In 2015, Dr. Jun Liu and Dr. Nigel Browning led a study at PNNL that
showed how tin and antimony segregate into different regions in an electrode. The
team at PNNL took this work further to uncover the processes taking place at
the interfaces between these regions and to find the origin of electrode
stability.

The team used experiments and theory to determine how the crystal structure evolves in
the antimony- and tin-rich regions during charging and discharging, as would be
seen in a battery. When a magnesium ion slides into the tin region and later
leaves, it does not cause the collapse seen in pure tin. When the ion moves
into the antimony-rich region, it becomes locked into the structure. This makes
the antimony portion a poor electrode, but it is the antimony region that keeps
the whole electrode stable.

The alloy electrode
delivers a high reversible capacity and remains stable over hundreds of rounds of discharging
and charging. Moreover, the team demonstrated that the alloy electrodes worked
well with other magnesium battery components, specifically the simple salt
electrolytes. This makes the alloy electrodes more attractive and opens a new
avenue to explore magnesium batteries.

This year-long research project was possible because
of the team's diverse expertise in chemistry, materials sciences, engineering,
and physics. "The Office of Science provided opportunities for us to work
together," said Shao. "They also provided state-of-the-art capabilities at the
EMSL user facility."

What's Next? The PNNL team is planning to put
the alloy into a full battery system and see how it performs. On a broader
scale, this work will help others understand interfaces in fuel cells, nuclear
reactors, and other energy storage and production systems.

Additional Information

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In short

71-character summary: Tin and antimony work together to make
innovative batteries more stable

1-sentence summary: Scientists at Pacific Northwest National
Laboratory found that the interface between antimony and tin stabilizes an
alloy electrode of interest for higher capacity magnesium batteries; this offers
insights into the interface and adds to the scientific foundation necessary for
new energy technologies.